1. Field of the Invention
The present invention relates to a dressing device for dressing an electrode tip of a welding gun mounted on a resistance welding machine.
2. Description of the Prior Art
A tip serving as an electrode part of a welding gun is worn away as it is pressurized and conducted with a current for its repetitive welding. Accordingly, successive use exerts bad influences on welding strength. As a result, it is necessary to periodically dress the tip. Referring then to FIGS. 5 and 6, such a dressing device for automatically dressing a tip will be described.
A base 1 has a rod 3 supported thereon and adapted to penetrate a driving device 2, said rod 3 including a partition wall 4 mounted thereon for halving the interior of a chamber of the driving device 2. Springs 5, 6 are respectively disposed between both side walls of the driving device 2, and the partition wall 4. The driving device 2 is adapted to be movable relative to the base 1 along the rod 3.
When a changeover valve 7 is moved to a position (a) then pressurized air from a compressor (not shown) flows through the changeover valve 7 from pipes 8, 9 and enters into one chamber (left side) of the driving device 2 via a pipe 10, whereby air pressure in the left chamber of the driving device 2 is increased to cause the driving device to be moved to the left along the rod 3. The leftward movement compresses the spring 6. With the movement of the driving device 2, a dresser body 11 also moves leftwardly causing a blade thereon to move into close contact with one tip (such as the left tip 48 in FIG. 2.) which prevents further leftward movement of the dresser body 11. The driving device 2 is further moved leftwardly while leaving behind the dresser body 11 at its position of contact with the tip, and once a difference between the relative positions of the driving device 2 and the dresser body 11 reaches a prescribed value, then a switch 12 is operated to open an air on-off valve 13. Thus, the air flows through the pipe 8, a pipe 14 and the on-off valve 13, and is fed to an air motor disposed in the dresser body 11 from a pipe 15. The driving device 2 stops its leftward movement when the partition wall 4 makes close contact with a bush 16 of the right chamber.
The air motor disposed in the dresser body 11 begins to be rotated by the compressed air from the pipe 15 to rotate the dresser blade, and when the inlet pressure P of the air supplied to the air motor reaches a reference value, then a timer 17 starts its operation. Namely, the timer permits the air motor to be normally rotated for a desired time to dress the tip. After lapse of the desired time, the changeover valve 7 is changed over by the timer 17 to a neutral position (b) to make the pressure difference between both chambers equal, thereby allowing the driving device 2 to be first moved to the right by the spring 6 to reduce the relative positional difference between the driving device itself and the dresser body 11. Thus, the switch 12 is switched off to close the on-off valve 13 and thereby rotation of the dresser blade is stopped. Then, also the dresser body 11 is separated from the tip owing to the movement of the driving device to the right, and is moved in the same direction accompanied by the driving device, and finally both the dresser body and the driving device return to the original state. Hereupon, the timer 17 is also reset for allowing the driving device to be returned to the initial state.
Moreover, in dressing the other tip (such as the right tip 51 in FIG. 2), the changeover valve 7 is moved to position (c) so as to move the driving device to the right side.
However, the prior dressing device suffers from some drawbacks: As the compressed air is supplied from the changeover valve, for example as pressure in the left chamber is increased as shown in FIG. 6, the driving device 2 is moved to the left on the base 1 and thereby the spring 6 is compressed as described above. There are thereupon instances where reactions (i.e., the restoring forces) of both the springs are increased in proportion to the movement of the driving device, and hence further leftward movement of the driving device is made either more difficult or impossible. Even if the movement of the driving device is made possible, the pressing force against the tip is changed due to a difference between the positions of the tip and the dresser body depending on the movement of the dresser body (and the large or small influence of the reaction of the spring 6). Accordingly, there are some cases according to circumstances wherein cutting or dressing of the tip is insufficient or excessive.